The present invention relates to a process for decreasing the content of organic-chemical substances in waste water by chemical oxidation, preferably in the presence of chlorine, and optionally additional alkaline thermal treatment of the waste water, and also to the use of the process for the treatment of waste water.
It is already known to subject waste water containing undesirable, dissolved or dispersed organic substances to oxidative chemical-thermal treatment, in which the organic substances are oxidized to carbon dioxide and water. Such a process is described, for example, in published German Patent Application No. DE 26 40 603, and is generally known under the name "wet oxidation".
The wet oxidation process is characterized by oxidation in the liquid phase. In order to achieve satisfactory degradation of the organic compounds and sufficient reaction rates, a certain oxygen concentration and high temperatures and pressures are required. This treatment method is therefore very expensive owing to its high energy consumption and the costly apparatus. Although the temperatures and pressures used may be reduced by using heavy-metal catalysts, this leads to subsequent contamination of the waste water with the catalytically active substances, which may make additional treatment steps necessary.
The contamination of bodies of water with organic substances is characterized by a number of physical-chemical parameters. These include the "chemical oxygen demand" (COD) as a measurement of the oxygen consumption upon chemical wet oxidation of the organic compounds dissolved or dispersed in a body of the water under defined conditions. The content of chemically oxidizable substances in waste water can thus be calculated by means of this parameter.
It is known from Japanese Patent Application 49/021951 (reviewed in Chemical Abstracts CA81(10): 54117j) to subject water containing organic substances to treatment with UV light and with active chlorine or hypochlorite, followed by subsequent treatment with activated carbon, in order to reduce the COD value. However, the use of UV light in the treatment of water is technically complex and expensive.
A process for decreasing pollutants producing COD and AOX in waste waters from bleaching plants from pulp production is known from DE-OS 36 20 980. The AOX is a further summation parameter for characterizing organic contamination in bodies of water. It stands for "adsorbable organic halogen compounds", and is determined as the portion of organic halogen compounds (X=Cl, Br, I) which can be adsorbed on activated carbon, with the total quantity adsorbed being converted to X=Cl.
The process of published German Patent Application No. DE 36 20 980 is preferably performed in three stages, with the strongly acidic solution from chlorine bleaching in the first stage having calcium ions added to precipitate chlorine compounds and hence to reduce the COD and the AOX. In a second stage, an additional AOX reduction takes place by means of thermal alkaline hydrolysis, which leads to partial de-chlorination of the non-precipitated adsorbable organic chlorine compounds. The pollutants producing COD and AOX which are still contained in the waste water after this process step are subjected to microbial degradation in a third stage.
Even this process for decreasing organic pollutants in waste water is very costly, and hence is unsatisfactory from the economic point of view.